test: fix non-deterministic compose context path

[crazy-max]

related to #1971

Since #1971 the context path is now absolute and although the tests run fine in our containerized environment, it wouldn't work running the tests locally, such as:

=== CONT  TestReadTargetsCompose
    bake_test.go:306: 
        	Error Trace:	X:/dev/neard/www/github/docker_org/buildx/bake/bake_test.go:306
        	Error:      	Not equal: 
        	            	expected: "/src/bake"
        	            	actual  : "/home/foo/buildx/bake"

[glours]

LGTM